How resistant is cotton to new races of fusarium wilt?

During the past nine years, a new race of Fusarium wilt known as FOV Race 4 has caused increasing damage to cotton in California’s San Joaquin Valley. In a paper published online in Agronomy Journal on Sept. 6, researchers now report the results of field studies that measured the ability of elite cotton breeding lines to resist this new pathogen.

Once Fusarium is introduced into crop production areas, it expands widely and is nearly impossible to eradicate from soil as no chemicals can control it. Instead, one of the most effective and low-cost methods to manage the disease is to plant Fusarium-resistant cotton.

But little is known so far about the genetic resistance in U.S. upland cultivars and germplasm to Race 4 and other FOV races, or how these races interact with other cotton pathogens, such as Verticillium and reniform nematode. Moreover, the complete host range of the new FOV races is not well understood. With such limited knowledge about the ecological factors that favor these races, cotton production across the United States could be at risk.

To begin filling this knowledge gap, researchers from the USDA-ARS and the University of California were involved in 2008, 2010, and 2011 in FOV host-plant resistance trials. The team evaluated advanced breeding lines from the Regional Breeders Testing Network (RBTN, http://www.cottonrbtn.com/) for disease resistance to FOV Race 1 and a second race, Race 4. Ten replicated field evaluations were conducted for each race on three different sites in the San Joaquin Valley.

During all three years, the elite breeding lines showed significant differences in their levels of resistance-response to FOV Races 1 and 4, as evidence by two measures of disease severity and overall plant survival. Studies of upland cotton lines also revealed interactions in FOV disease response between elite germplasm lines, FOV Races 1 and 4, and evaluation sites.

Elite lines selected from breeding programs in the states of Alabama, Arkansas, Louisiana, and Mississippi showed at least a moderate level of tolerance to both Fusarium races. Thus, the studies identified germplasm or genetic sources of FOV resistance that could be useful to breeders for improving cotton.

However, based on these evaluations, many of the entries in the current elite upland germplasm pools used by public plant breeders may be more susceptible than expected to some FOV races, and sources of acceptable levels of resistance may be limited when tested under infestation levels that resulted in only 5 to 35% plant survival in susceptible check cultivars. Continued development of cotton germplasm with wide adaptability, improved host-plant resistance, and enhanced fiber quality is essential for the long-term survival of the U.S. cotton industry.